The invention relates to an antenna coupling device for testing mobile telephones. An antenna coupler is a component of a test structure for servicing and development of mobile telephones.
When testing mobile telephones or mobiles, a plurality of properties or parameters are investigated, which can be loosely categorized as transmission and reception measurements. In transmission measurements for example, among other things a measurement of the phase error, of the frequency error, of the power and of the spectrum is carried out. In reception measurements bit error rates are measured. With respect to the technical background of tests of radio telephones, reference is made to chapters II and III of the book xe2x80x9cGSM-Technik und Mexcex2praxisxe2x80x9d by Siegmund Redl and Mathias Weber, Franzis-Verlag 2, second edition, Poing, 1995 from the series Funkschau Technik.
In order to enable measurement which approximates as closely as possible to practical use of the mobile telephone, the mobile telephone to be tested, (also named test item or DUTxe2x80x94device under test) should transmit and receive HF (=high frequency=radio frequency=RF) power via its antenna. If the transmission properties to the test item are known and sufficiently constant, the power received or transmitted by the test item can be calculated from the measured values. In the measuring method known per se with an antenna on the measuring appliance for coupling with the antenna of the test item, the measurement results are substantially influenced by the following parameters:
(i) space between both antennas; (ii) environment of the antennas (measuring appliance, laboratory installation, walls, people), due to reflections; (iii) interference from irradiating interfering transmitters; and (iv) HF properties of the antenna (radiation characteristics via solid angle and frequency).
Consequently measurements are only possible in large screened chambers, which are lined with HF-absorbent materials. Furthermore, the entire measuring apparatus must be located outside the chamber. In order to use the test item manually the person must enter the chamber and leave it again before measurement can be continued.
Miniature measuring chambers designed as boxes are also known in prior art. In these a basic disadvantage that in order to use the test item the box must be opened and closed again. Moreover, such a box is inconvenient, complex and expensive to manufacture. Transmission of the HF according to prior art is carried out in the interior of the box by a coupling mechanism, which is pushed over the antenna of the test item (model CB Z10 of Rohde and Schwarz), or by a dipole antenna with a coarse x- and y- positioning of the test item (model AH 5911 of ANDO) by mounting the test item on a co-ordinate system. In the first known type of transmission a disadvantage is that this is not suitable for test items with an antenna integrated in the casing or with an antenna too large for the coupling mechanism. In the second type, positioning of the test item is imprecise, as the latter cannot be securely fixed. Finally, in both known types of HF transmission the frequency range of the coupler is restricted to an undesirable degree by the structure, and the coupling attenuation for many test items is so high that the dynamics of the measuring appliance are not sufficient.
Therefore the object underlying the present invention is to avoid the disadvantages of prior art, and in particular to provide a simply constructed antenna coupler which has a low coupling attenuation for the plurality of commercially available radio telephones.
The object of the invention is achieved by providing an antenna coupler for testing mobile telephones, wherein the antenna coupler comprises:
a printed circuit board, upon the upper side of which there is formed by strip transmission line technique at least one antenna element; and
an accommodating element for a mobile telephone, secured above the printed circuit board.
According to an advantageous embodiment of the invention, the printed circuit has a first and a second antenna element, the first antenna element being provided for a first frequency range and the second antenna element for a second frequency range. The first frequency range is 1710-1900 MHz (DCS 1800 and DCS 1900 system) and the second frequency range is 880-960 MHz (GSM). In this way mobiles of the D1, D2, E and US-network, and further networks operating in this frequency range, can be tested with one antenna coupler.
More advantageously, the upper side of the printed circuit board, particularly its external area, consists of conductive material which is earthed. This provides a certain screening of external fields, without the necessity for a space-consuming box. Furthermore, access to the mobile during testing is ensured in this way.
Preferably, the first antenna element is a dipole antenna and the second antenna element is a slot antenna. In a preferred arrangement the first antenna element is disposed parallel to and substantially beneath the antenna of the mobile telephone on the upper side of the printed circuit board, and the second antenna element is disposed orthogonally to the antenna of the mobile telephone and substantially centrally under the body of the mobile telephone. After a plurality of experimental tests on previously known mobiles it was discovered that the type of antennas and this arrangement for testing in two frequency ranges enables a particularly favorable coupling attenuation.
Both antenna elements are preferably combined via a diplexer to a connection with a test device, so that the antenna coupler can be connected via a single connection to a test device for a plurality of frequency ranges.
According to a preferred embodiment of the invention the diplexer is formed on the underside of the printed circuit board, like the antenna elements in strip transmission line technology, so that an extremely cost-effective and compact structure can be produced.
In a further advantageous embodiment of the invention the underside of the printed circuit board is surrounded by a screening casing, in order to eliminate interfering influences from the environment.
More advantageously, the receiving element for the mobile telephone is a mobile universal holder made of non-metallic material, particularly plastic. Thus the antenna coupler is suitable for all mobile types available on the market, and the test item to be tested is in each case precisely positioned, so that a high repeatability or reproducibility of the measurement values can be achieved. On this basis, software correction methods specific to the mobile telephone can be used, in order to take account of the specific HF transmission properties.